91. And fulfill the Covenant of Allah (Bai’a: pledge for Islam) when you have covenanted, and break not the oaths after you have confirmed them, and indeed you have appointed Allah your surety. Verily! Allah knows what you do.

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Tafsir Ibn Kathir Volume 5.

This is one of the commands of Allah, to fulfill covenants, keep promises and to fulfill oaths after confirming them. Thus Allah says:

“By Allah, if Allah wills, I will not swear an oath and then realize that something else is better, but I do that which is better and find a way to free myself from the oath”. According to another report he said: “and I offer penance for my oath”.

There is no contradiction at all between all of these texts and the Ayah under discussion here, which is:

﴿وَلاَ تَنقُضُواْ الاٌّيْمَـنَ بَعْدَ تَوْكِيدِهَا﴾

“and do not break the oaths after you have confirmed them”

Because these are the kinds of oaths that have to do with covenants and promises, not the kind that have to do with urging oneself to do something or preventing him from doing something.